Use of thermal insulation in apparel is well known, with conventional materials consisting of batting, foam, down and the like. By way of example, insulation for footwear articles is known to include such materials as leather, felt, fleece, cork, flannel, foam, high loft batting and combinations thereof. A disadvantage of conventional insulating materials is that achieving high levels of insulation requires the use of a relatively large thickness of material. For example, adequate insulation in conventional footwear for sub-freezing temperatures can be on the order of several centimeters thick. In many applications for footwear used outdoors, the provision of a large thickness of material is impractical especially in apparel items for work or sport. In these activities, there often exists requirements of agility, surefootedness and firm traction for the feet. Too great a thickness of insulation introduces the possibility of relative motion between the body and the item being worn and hence an insecure contact with the ground. The aesthetics of an article may also be affected by added thickness and users may be averse to wearing bulky items of apparel which have an unflattering or unfashionable appearance. Additionally, the added bulk of conventional insulation tends to impact comfort and stiffness of the footwear to the wearer.
The art is replete with footwear constructions targeting adding insulation, particularly in the toe region, to enhance comfort and warmth of the toes. Several exemplary patents in the prior art are described in more detail below.
U.S. Pat. No. 4,055,699, in the name of Hsiung teaches a multi-layer insole for an article of footwear to insulate the foot from cold which is sufficiently thin to insulate without changing fit. The insole is a multi-layered laminate having a thin soft fabric layer laminated to the top of an open cell foam layer, a dense cross-linked polyolefin layer laminated to the foam layer, and an aluminum coated barrier layer of polymeric material laminated to the bottom of the cross-linked polyolefin layer. It is taught, however, that the insole is compressible and the open celled layer tends to pump air as body pressure is alternately applied, circulating warm air around the side of the foot within the shoe. Additionally, to increase insulation it is taught to increase the thickness of the open-celled layer.
The thermal conductivity of conventional insulation material used for apparel and footwear is generally greater than that of air which has a thermal conductivity of about 25 mW/m K at 25° C. In the case of high density materials such as neoprene foam, high conductivity may result from conduction by the solid component, or in materials of intermediate density, a combination of conduction, convection, and radiation mechanisms may result in higher effective conductivity. Conventionally, to substantially increase the level of insulation, a substantial increase in thickness of insulation material is required, which has the above-stated disadvantages such as changing the fit of an article.
U.S. Pat. No. 7,118,801, in the name of Ristic-Lehmann, is directed to material comprising aerogel particles and a polytetrafluoroethylene binder is formed having a thermal conductivity of less than or equal to 25 mW/m K at atmospheric conditions. The material is moldable or formable, having little or no shedding of filler particles, and may be formed into structures such as tapes or composites, for example, by bonding the material between two outer layers. These composites may be flexed, stretched, or bent without significant dusting or loss of insulating properties.
U.S. Pat. No. 7,752,776, in the name of Farnworth, is directed to articles of apparel comprising insulating components having insulating structures with low thermal conductivity. The insulating components have an insulating structure comprising a gas impermeable envelope and a porous material contained within the envelope where the insulating structure has a thermal conductivity of less than or equal to 25 mW/m K.
U.S. Pat. No. 7,603,796, in the name of Johnson, Jr. is directed to a boot, such as a hunting boot, having an oversized toe box within which a layer of cold weather insulating material of increased thickness is provided. According to the invention, a boot is provided with an oversized toe box where substantially more conventional high bulk, cold weather insulation is provided than a boot having a conventional toe box. Such oversized features have significant limitations in comfort, agility and appearance of boot for the wearer due to the larger size and bulk in the toe region.
US Pub. No. 2007/0128391, in the name of Giacobone, is directed to an insulating component having a layer of insulating material and a sealed envelope around the layer of insulating material, the envelope being made of elastomer material. The envelope is sealed by a peripheral weld. In a particular exemplary embodiment, the insulating component is part of an article of footwear, in which the component is positioned between an outer layer and an inner layer of a liner and is assembled to the upper by a seam along the peripheral weld.
European Patent Application Publication No. 0736267, to Pfister et al., is directed to a heat insulating footwear cap and footwear incorporating the cap. The heat insulating cap is lined with and consists of an air storing material which is so compression resistant that during normal use of the footwear the air storing capacity, and thus its heat insulating capacity, is maintained. Again, significant limitations exist with this footwear construction due to the added bulk in the toe region.
While these patents generally teach providing additional insulation incorporated within already highly insulated footwear, they do not provide for footwear articles which delivers agility, surefootedness and firm traction, along with attractive aesthetics and comfort of conventional uninsulated or minimally insulated shoes and boots (e.g., having upper thermal resistance values 0.18 m2° C./W or less).
There is a need for footwear which provides warmth without substantially changing the fit, appearance and comfort of a footwear article, whether a conventional insulated or uninsulated footwear article. There has been a long-felt need for low bulk insulating materials uniquely oriented in footwear articles to achieve such desired footwear.